Net Income Per Share - Reconciliation between Basic and Diluted Weighted Average Shares Outstanding (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 62,070	$ 56,951
Net income attributable to unvested awarded performance stock	(1,451)	(1,363)
Net income attributable to common shares	$ 60,619	$ 55,588
Weighted average number of common shares outstanding - basic (in shares)	138,793	142,777
Less unvested awarded performance stock included in weighted average number of common shares outstanding - basic (in shares)	(3,245)	(3,417)
Weighted average number of common shares outstanding for basic earnings per common share (in shares)	135,548	139,360
Dilutive effect of stock options (in shares)	1,392	2,127
Weighted average number of shares outstanding - diluted (in shares)	136,940	141,487
Basic (in dollars per share)	$ 0.45	$ 0.40
Diluted (in dollars per share)	$ 0.44	$ 0.39